you could but you would want to add lots of yeast nutrient. and there is some chance, if Chris White is correct, that you will grow yeast with a diminished ability to metabolize maltose which isn't good.

Are you an all grain brewer?

If so consider adding an extra lb or two to the next brew and pulling off an extra gallon or so of wort. boil/dilute to 1.030ish and pressure can or freeze. If you freeze boil again before use.

This is free! (or nearly) I tend to run around 70% extraction eff so I can just put some extra water through my grain bed and get a few quarts of starter wort.

I have frozen wort in my freezer for just this purpose. I follow a modified Morticai approach in that rather than add extra grains I simply set my grain bag off to the side and collect the last bit that drains out. Partial mash, so I'm only dealing with 5 - 7 lbs of grain and I'm not really sparging.

When it's time to get a starter going, there is almost no effort involved at all.

A second running starter wort is an awesome idea. Run it into quart jars then 15 minutes in the pressure canner. I might have to try that.

works a treat.

you can take it a step further and can 1.5 quarts in a 2 quart mason jar. Then you spray with star san, pop the lid, pitch the yeast, set the lid back on gently and walk away. I don't use a stir plate so this works for me. If you use a stir plate it wouldn't really gain you anything and those .5 gallon ball jars are pricey.

Got it thanks for the ideas. I normally use dry yeast but just pitched a vial of white labs for the first time in a while. About a 40 hour lag time into 1.045 wort...

I will often pull a starter sized portion of wort out of the boil half way through, chill and pitch into that on brew day and pitch it the next day in a pinch.

It takes a lot less time to chill 1-1.5 quarts of wort than the whole batch. and I always chill my wort from ~80 to pitching temps in the ferm fridge overnight anyway so it works well for me. This is the only time I pitch the entire starter instead of crashing and decanting.